High-pressure gas discharge lamp

ABSTRACT

A high-pressure gas discharge lamp having a discharge vessel provided with materials, which during operation of the lamp are present in a gaseous and/or vapor state in which at least one of these materials during operation of the lamp provides a saturated vapor, and having an electrode placed at one end of the discharge vessel in which the discharge vessel at said end is provided with an external coating. The coating consists of a first film located on the wall of the discharge vessel and comprising a black or dark grey material having a high melting point and low vapor pressure (for example, carbon), and a second film located on the first film and comprising a white or substantially white material having a high melting point and a low vapor pressure (for example, zirconium oxide).

United States Patent 1191 Beyer et a1.

1111 3,842,304 1451 Oct. 15,1974

1 1 HIGH-PRESSURE GAS DISCHARGE LAMP [75] Inventors: Louis Benjamin Beyer; Gerardus Antonius Petrus Maria Cornelissen; Antonius Jozephus Gerardus Cornelis Driessen; Cornelis Adrianus Joannes Jacobs; Gerardus Henricus Maria Siebers, all of Emmasingel, Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

221 Filed: Apr. 30, 1973 21 Appl. No.: 355,905

[30] Foreign Application Priority Data May 16, 1972 Netherlands 7206559 [52] US. Cl 313/44, 117/216, 117/221, 117/124 A, 313/47, 313/220, 313/221 [51] Int. Cl. H0lj 61/52 [58] Field of Search 313/44, 47, 220, 221; 117/216, 221,124 A [5 6] References Cited UNITED STATES PATENTS 3,513,344 5/1970 Larson 313/47 Primary Examiner1-1erman Karl Saalbach Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or Firm--Frank R. Trifari 5 7 ABSTRACT A high-pressure gas discharge lamp having a discharge vessel provided with materials, which during operation of the lamp are present in a gaseous and/or vapor state in which at least one of these materials during operation of the lamp provides a saturated vapor, and having an electrode placed at one end of the discharge vessel in which the discharge vessel at said end is provided with an external coating The coating consists of a first film located on the wall of the discharge vessel and comprising a black or dark grey material having a high melting point and low vapor pressure (for example, carbon), and a second film located on the first film and comprising a white or substantially white material having a high melting point and a low vapor pressure (for example, zirconium oxide).

7 Claims, 1 Drawing Figure HIGH-PRESSURE GAS DISCHARGE LAMP The invention relates to a high-pressure gas discharge lamp having a discharge vessel provided with materials which during operation of the lamp are present in a gaseous and/or vapor state, in which at least one of these materials during operation of the lamp supplies a saturated vapour, and provided with an electrode placed at one end of the discharge vessel, the discharge vessel at said end being provided with an external coating. Furthermore the invention relates to a method of applying an external coating on a lamp of this kind.

A lamp of the kind described above comprises unevaporated material in the operating condition. This unevaporated material is generally present at that area in the lamp having the minimum temperature. The said minimum temperature then determines the vapour pressure for the relevant material. In many cases the area having the minimum temperature is present on a portion of the wall of the discharge vessel located around and behind the electrodes. Control of the temperature around and behind the electrodes, for example, by proportioning the electrode space is generally possible with difficulty due to the presence of current supply conductors which are passed through the wall of the discharge vessel at that area. Passing of a current supply conductor may be effected, for example, by a vacuum-tight sealing in the material of the discharge vessel. Such a sealing has considerable limitations in practice relative to the proportioning of the electrode space as well as to the reproducibility of the chosen form of the electrode space. A result of the insufficient control of the minimum temperature in the lamp is that the lamps mutually exhibit considerable differences as regards light output and spectral distribution of the emitted radiation.

A solution of the problem described is possible if the temperature of the wall of the discharge vessel around and behind the electrode is increased, so that the area having the minimum temperature is transferred to the satisfactorily proportioned intermediate portion of the discharge vessel. in order to obtain a temperature increase of the portion of the wall of the discharge vessel located around and behind the electrode it is known to provide an external coating on said portion of the wall of the discharge vessel so that at least part of the incident radiation (ultraviolet, visible and infra-red radiation) is reflected or absorbed.

As is known thin reflecting metal films, for example, consisting of gold or silver alloys may be used as an external coating. Such metal films have, however, the drawback that they are not resistant to temperatures of more than approximately 700C.

Furthermore itis known to provide reflecting white oxide films, for example, of zirconium oxide, titanium oxide, or aluminum oxide as external coatings. A drawback of these oxide films is that they can only be provided with difficulty. In order to realize the desired effeet the oxide films must be relatively thick so that generally a poor adhesion to the wall of the discharge vessel is obtained.

A drawback of both the metal films and the white oxide films is that they reflect a large part of the incident radiation without leading to an increase of the temperature of the wall of the discharge vessel.

Finally it is known to use black films, for example, consisting of carbon in order to increase the temperature of the portion of the wall around the electrode. Al-

though these black films absorb a very large part of the incident radiation, they have the drawback that they emit the greater part of the absorbed radiation again.

The object of the invention is to provide an external coating of the discharge vessel of a high-pressure gas discharge lamp with which higher temperatures of the coated portion of the wall of the discharge vessel can be obtained as compared with the known coatings and in which the above-mentioned drawbacks do not occur.

According to the invention a high-pressure-gas discharge lamp of the kind described in the preamble is characterized in that the coating consists of a first film located on the wall of the discharge vessel and comprising a black or dark-grey material having a high melting point and a low vapor pressure, and of a second film located on the first film and comprising a white or substantially white material having a high melting point and a low vapour pressure.

It has been found that in a lamp according to the invention a considerably larger increase of the temperature of the wall section around and behind the electrode is obtained than in the case of the known reflecting or absorbing coatings. Even at very high mean wall loads (for example, 25 W/sq. cm) it has been found that no unevaporated material is present in the electrode space, which indicates that the coldest spot in the lamp is located on the intermediate part of the wall of the discharge vessel. The use in a lamp according to the invention of a black or dark-grey material in the first film located directly on the wall of the discharge vessel has the advantage that substantially all incident radiation is absorbed. This leads to a higher temperature increase of .the wall material itself than in the case of total reflection of the incident radiation. In a lamp according to the invention the use of a second, reflecting, and hence poorly emitting film prevents the heat absorbed in the first film from being lost by radiation to the exte- I'lOI'.

Since the combination according to the invention of a black absorbing film and a white reflecting film has a considerably higher temperature increasing action than the known single films consisting of absorbing or reflecting material, it is sufficient for the films in a lamp according to the invention to be thinner than in the case of the known lamps in order to realize the same temperature increase. This is an important advantage because, as is known, thin films generally adhere better than thick films. In addition it has been found that white films generally adhere better to a black film than to the material of the discharge vessel which often consists of quartz glass. Thus, in a lamp according to the invention the second white film may be used in a larger thickness while maintaining a satisfactory adhesion than is possible in the known lamps using a white film only. A lamp according to the invention furthermore has the advantage that a more even temperature distribution on the wall of the discharge vessel is obtained so that large temperature differences along the wall are avoided.

In this description and in the Claims a black or darkgrey material is understood to mean a material which has a reflection coefficient of less than or equal to 0.2. A white or substantially white material is understood to mean a material which has a reflection coefficient or more than or equal to 0.5. If these materials are provided in a film on the discharge vessel the reflection coefficient of the film may still deviate slightly from that of the materials themselves. Of course it is necessary that the materials to be used for both films have a high melting point (for example, more than l,000 K).

It is likewise necessary that these materials have a low vapor pressure (for. example, less than Torr at 1,200K).

As a material for the first film carbon, carbides (for example, tungsten carbide) silicates (for example, tungsten silicate or molybdenum silicate), borates (for example, molybdenum borate) or mixtures of the said materials are preferably used, and one or more ceramic oxides (for example, calcium oxide, magnesium oxide, zirconium oxide, aluminium oxide and thorium oxide) are chosen as materials for the second film.

In one preferred embodiment of a lamp according to the invention the firstfilm mainly consists of carbon or graphite and the second film mainly consists of zirconium oxide. In fact, optimum results are obtained with these materials. It has also been found that these materials can easily be provided in the form of satisfactorily adherent films.

The external coating in a lamp according to the invention is preferably located around the electrode and it extends to not more than 5 mms beyond the end of the electrode facing the discharge. If the coating extended still further to the intermediate part of the discharge vessel, a too large portion of the useful radiation emitted by the lamp would be absorbed.

A lamp according to the invention may be, for example, a high-pressure sodium vapor discharge lamp having a discharge vessel of, for example, polycrystalline aluminium oxide comprising sodium, mercury and a rare gas. In such a lamp sodium and mercury are present in an excess so that during operation of the lamp saturated sodium and mercury vapours are present. A satisfactory control of the minimum temperature in this lamp and hence a satisfactory control of the sodium and mercury vapor pressures is of great importance for a satisfactory operation of the lamp.

The invention may be very advantageously used in a high-pressure gas discharge lamp whose discharge vessel consists of quartz glass or hard glass and is filled with mercury, one or more rare gases and one or more metal halides. The coating is provided around the electrode (and possibly also around at least part of the sealing of the current supply conductor) which coating extends to not more than 2 mms beyond the end of the electrode facing the discharge. These lamps often employ halides which do not readily evaporate (for example, sodium iodide and the iodides of rare earth metals). These halides are then present in the lamp in an excess. In such a halide-containing lamp according to the invention it is particularly advantageous that in addition to a shift of the coldest spot in the lamp to the intermediate part of the discharge vessel, so that reproducible lamps are obtained, also an increase occurs of the minimum temperature prevailing in the lamp. Consequently a larger quantity of the halide not readily evaporating can be introduced into the discharge so that the efficiency of the lamp and the spectral distribution of the emitted radiation can be favourably influenced.

Generally it is preferred to provide the two ends of the discharge vessel of a lamp according to the invention with an external coating in order to increase the (and possibly behind) the two electrodes.

An external coating consisting of a first film of carbon and a second film of zirconium oxide is preferably provided on a high-pressure gas discharge lamp according to the invention by means of a method according to the invention in which the end of the discharge vessel in the vicinity of the electrode is coated with a first film of a graphite suspension, which film is coated after drying with a second film consisting of a suspension of zirconium oxide in a suspension agent comprising a solvent and a binder whereafter the coating thus obtained is dried and subsequently heated in air at a temperature of 250 500 C. The suspension films may be provided, for example, by immersion or spraying or brushing. When heating the coating at 250 500C the suspension agent and the binder are removed and a satisfactory adhesion of the films to each other and to the discharge vessel is obtained.

In a method according to the invention it is advantageous to use a colloidal solution of graphite in water for the first film (for example, the product known under the trademark aquadag) and for the second film a suspension of zirconium oxide in an organic solvent (for example, butylacetate) comprising an organic binder (for example, nitrocellulose). By using an aque ous suspension for the first film and an organic suspension for the second film a mixture of the two films, which results in an unwanted grey discoloration of the white zirconium oxide film, is substantially excluded.

The invention will now be further described with reference to a drawing.

The drawing shows a high-pressure gas discharge lamp according to the invention which is suitable for a power of 2,000 W. The vquartz glass discharge vessel consists ofa cylindrical section 1, which has an external diameter of approximately 30 mms. The two ends of the section 1 adjoin conical sections 2 and 3 which are closed by pinches 4 and 5, respectively. Current supply elements 6 and 7 are sealed in a vacuum-tight manner in the pinches 4 and 5, respectively. These current sup ply elements are connected within the discharge vessel to electrodes 8 and 9, respectively, which consist of tungsten filaments secured to tungsten pins. The distance between the two electrodes 8 and 9 is approximately mms. In practice the lamp is usually mounted in an evacuated or inert gas-filled outer envelope (not shown in the drawing). The discharge vessel is filled with mg Hg, 6 mg Dy, 12 mg Hgl 5 mg Tll, 3 mg Cs] and 0.3 mg Nal and furthermore with argon up to a pressure of 20 Torr.

Dysprosium iodide which is formed during operation of the lamp and also sodium iodide are present in an excess, that is to say, during operation a saturated vapor of dysprosium iodide and of sodium iodide is formed and still unevaporated dysprosium iodide and sodium iodide are present. This unevaporated iodide is then present at those areas on the wall of the discharge vessel which have the lowest temperature. In order to avoid that the area of the lowest temperature is present on that part of the wall of the discharge vessel surrounding the electrode 8 (the conical section 2 and a part of the pinch 4) an external coating 10 is provided on the part of the discharge vessel located around and behind the electrode 8. The coating 10 is located on part of the pinch 4 and furthermore extends across the conical section 2 up to several millimetres before the tip of the electrode 8. The coating consists of a first film of carbon directly located on the quartz glass, which film is provided with the aid of an aquadag" suspension, and furthermore a second film of zirconium oxide located on the first film. The zirconium oxide film is provided with the aid of a suspension of 150 grs of Zr0 in 150 grs of butylacetate which comprises 5 percent by weight of nitrocellulose. A coating 11, which is entirely analogous to the coating 10, is provided around the electrode 9.

It has been found that during operation of the lamp described above the area having the minimum temperature on the wall of the discharge vessel is found on the cylindrical section 1. During operation of the lamp unevaporated iodide is observed on the cylindrical part 1 approximately at the height of the arrow 12 when the lamp is vertically operated. Furthermore a lamp current of 9.7 A, a lamp voltage of 230 V, a luminous flux of approximately 170.000 lm, a colour temperature of the emitted radiation of approximately 6500 K and a color rendering index Ra of more than 85 were measured on the lamp.

What is claimed is:

1. In a high-pressure gas discharge lamp comprising a discharge vessel provided with materials which during operation of the lamp are present in a gaseous and/or vapor state, at least one of said materials during operation of the lamp supplying a saturated vapor, said lamp provided with an electrode placed at one end of the discharge vessel, the discharge vessel at said end being provided with an external coating, said coating consisting of a first film located on the wall of the discharge vessel and comprising a black or dark-grey material having a high melting point and a low vapor pressure, and of a second film located on the first film and comprising a white or substantially white material having a high melting point and a low vapor pressure.

2. A high-pressure gas discharge lamp as claimed in claim 1, wherein the first film mainly consists ofat least.

one of the materials carbon, carbides, silicates-and borates and that the second film mainly consists of at least one ceramic oxide.

3. A high-pressure gas discharge lamp as claimed in claim 1, wherein the first film mainly consists of carbon and the second film mainly consists of zirconium oxide.

4. A high-pressure gas discharge lamp as claimed in claim 1 wherein the coating is located around the electrode and extends to not more than 5 mms beyond the end of the electrode facing the discharge. I 5. A high-pressure gas discharge lamp as claimed in claim 1 in which the discharge vessel consists of quartz' glass or hard glass and is filled with mercury, one or more rare gases and one or more metal halides",- wherein the coating is located around the electrode and extends to not more than 2 mms beyond the end of the electrode facing the discharge. 6. A method of providing an external coating on a high-pressure gas discharge lamp as claimed in claim3 wherein the end of the discharge vessel in the vicinity of the electrode is coated with a first film of a graphite suspension, the first film being coated after drying with a second film consisting of a suspension of zirconium oxide in a suspension agent comprising a solvent and a binder, the coating thus obtained after drying bein heated in air at a temperature of 250 500C. 7. A method as claimed in claim 6, wherein a colloidal solution of graphite in water is used for the first film and a suspension of zirconium oxide in an organic solvent comprising an organic binder is used for the second film. 

1. In a high-pressure gas discharge lamp comprising a discharge vessel provided with materials which during operation of the lamp are present in a gaseous and/or vapor state, at least one of said materials during operation of the lamp supplying a saturated vapor, said lamp provided with an electrode placed at one end of the discharge vessel, the discharge vessel at said end being provided with an external coating, said coating consisting of a first film located on the wall of the discharge vessel and comprising a black or dark-grey material having a high melting point and a low vapor pressure, and of a second film located on the first film and comprising a white or substantially white material having a high melting point and a low vapor pressure.
 2. A high-pressure gas discharge lamp as claimed in claim 1, wherein the first film mainly consists of at least one of the materials carbon, carbides, silicates and borates and that the second film mainly consists of at least one ceramic oxide.
 3. A high-pressure gas discharge lamp as claimed in claim 1, wherein the first film mainly consists of carbon and the second film mainly consists of zirconium oxide.
 4. A high-pressure gas discharge lamp as claimed in claim 1 wherein the coating is located around the electrode and extends to not more than 5 mms beyond the end of the electrode facing the discharge.
 5. A high-pressure gas discharge lamp as claimed in claim 1 in which the discharge vessel consists of quartz glass or hard glass and is filled with mercury, one or more rare gases and one or more metal halides, wherein the coating is located around the electrode and extends to not more than 2 mms beyond the end of the electrode facing the discharge.
 6. A method of providing an external coating on a high-pressure gas discharge lamp as claimed in claim 3 wherein the end of the discharge vessel in the vicinity of the electrode is coated with a first film of a graphite suspension, the first film being coated after drying with a second film consisting of a suspension of zirconium oxide in a suspension agent comprising a solvent and a binder, the coating thus obtained after drying being heated in air at a temperature of 250* - 500*C.
 7. A method as claimed in claim 6, wherein a colloidal solution of graphite in water is used for the first film and a suspension of zirconium oxide in an organic solvent comprising an organic binder is used for the second film. 